The present invention relates to a method of cleaning an ultrapure water supply system, and more particularly, to a method of cleaning a system for supplying ultrapure water used in semiconductor manufacturing process etc.
As for ultrapure water used in the cleaning step of semiconductor manufacturing process or the like, an ultrapure water supply system is known which circulates ultrapure water between an ultrapure water production apparatus and a point of use. In such a system, the ultrapure water produced by the ultrapure water production apparatus is supplied through a pipe to the point of use, and the remaining part of the ultrapure water that was not used in the point of use is returned through a different pipe to the ultrapure water production apparatus.
The ultrapure water used in the semiconductor manufacturing process or the like is required that it should not contain fine particles, organic matter and inorganic matter. Specifically, the ultrapure water is expected to meet requirements of, for example, the resistivity being 18.2 Mxcexa9xc2x7cm or more, fine particles contained being 1/mL or less, viable cells contained being 1/L or less, total organic carbon (TOC) contained being 1 xcexcg/L or less, silica contained being 1 xcexcg/L or less, metals contained being 1 ng/L or less, and ions contained being 10 ng/L or less.
Thus, also in the above ultrapure water supply system, the ultrapure water supplied to the point of use must satisfy these water quality requirements. However, external fine particles or particles produced inside the system can become mixed with the ultrapure water, causing deterioration of the water quality.
For example, as the ultrapure water keeps circulating through the ultrapure water supply system, the surfaces of a filtration membrane, pipes, etc. of the system are worn away, and the worn-off substances (fine particles) become mixed with the ultrapure water. Also, where the system has been stopped for a long term, fine particles such as dead bacteria or iron dust become mixed with the raw water or ultrapure water stagnating in the system. Further, in the case of a newly installed ultrapure water supply system, various kinds of fine particles remain stuck on the surfaces of the filtration membrane and pipes which are component elements constituting the system. Also, while the system is under construction, fine particles such as dust in the air, silica and aluminum enter the system and adhere to various parts of the system. When the newly installed system is set in operation, therefore, such fine particles adhering to the interior of the system become mixed with the ultrapure water circulating through the system.
Thus, in order to remove such fine particles, the ultrapure water supply system needs to be cleaned prior to the start of operation as well as at regular intervals of time. Conventionally, warm water or water containing hydrogen peroxide is used to clean the system. However, in the case of warm water, for example, the cleaning capability is low and it is probable that fine particles in the system cannot be satisfactorily removed. In Unexamined Japanese Patent Publication (KOKAI) No. 7-195073 is proposed a cleaning technique using alcohol with high cleaning capability. To remove fine particles to a satisfactory level, however, it is essential to use alcohol having relatively high concentration (about 10 to 80%). Since alcohol, if left in the system, deteriorates the quality (increases the TOC) of the ultrapure water produced by the system, a residual alcohol removing operation (rinsing) must be performed, which, however, prolongs the overall cleaning time.
Such a long cleaning time lowers the operating efficiency of the ultrapure water supply system as well as of the plant using the system, and therefore, the cleaning time should desirably be shortened. Especially in the case of a newly installed ultrapure water supply system, a set-up or preparatory time required for the system to produce ultrapure water meeting the requirements is usually as long as a whole month, and accordingly, there has been a demand for techniques capable of shortening the set-up time.
A cleaning method is also known in which, when a filtration membrane constituting the ultrapure water production apparatus is manufactured or attached to the apparatus, the filtration membrane is cleaned using ultrapure water to an extent such that the resistivity and TOC of the ultrapure water fall within respective allowable ranges. With this method using ultrapure water, fine particles that affect the resistivity or TOC can be removed, but it is probable that other kinds of fine particles are not satisfactorily removed.
An object of the present invention is to provide a cleaning method capable of satisfactorily cleaning an ultrapure water supply system and component elements thereof in a short period of time.
To achieve the object, the present invention provides a cleaning method for cleaning at least part of an ultrapure water supply system having an ultrapure water production apparatus connected to a point of use of ultrapure water via a passage. The cleaning method of the present invention comprises the steps of: (a) changing surface potential of fine particles present in the at least part of the ultrapure water supply system; and (b) discharging the fine particles from the at least part of the ultrapure water supply system to outside.
Fine particles in the ultrapure water supply system can occasionally be charged with electricity. If the surface potential of such charged fine particles is opposite in polarity to that of elements constituting the system, the fine particles adhere to the system elements due to electrostatic attractive force acting between the system elements and the fine particles, making it difficult to remove the fine particles. In the method according to the present invention, the surface potential of the fine particles is changed, preferably into the same polarity as that of the system elements, to eliminate the electrostatic attractive force acting between the fine particles and the system elements, preferably to produce electrical repulsive force between the fine particles and the system elements, so that the fine particles can be removed with ease. While in this state, ultrapure water, for example, is caused to flow through the system, thereby to discharge the fine particles from the system to outside. According to the present invention, therefore, the whole or appropriate part of the ultrapure water supply system can be cleaned satisfactorily in a short time. Also, it is possible to shorten the set-up time of a newly installed system.
Preferably, in the step (a), the fine particles are made to contact with a basic or alkaline solution or a solution of surfactant.
According to this preferred embodiment, by making the fine particles contact with the basic solution or the solution of surfactant, the surface potential of the fine particles can be changed without fail. Also, even if the solution (cleaning liquid) used has a low concentration of base or surfactant, the solution can produce a satisfactory effect of changing the surface potential of the fine particles. Where a low-concentration solution is used for the cleaning, the constituent of the solution remaining in the cleaned system, that is, the base or the surfactant, is small in quantity. As a consequence, there is a small possibility that the TOC of ultrapure water produced in the system after the cleaning will increase due to the residual constituent of the solution. Also, in cases where additional cleaning (rinsing) is performed using ultrapure water, for example, to remove the residual constituent of the solution, the cleaning time may be short in length, thus permitting the system cleaning as a whole, which includes the cleaning by means of the cleaning liquid (solution) and the cleaning (rinsing) by means of the ultrapure water, to be completed in a short period of time. In the step (b) of the preferred embodiment, the basic solution or the solution of surfactant, which was made to contact with the fine particles, is merely discharged from the ultrapure water supply system, for example. Alternatively, after the solution is discharged, ultrapure water for rinsing is introduced into the system and then the fine particles are discharged from the system to outside together with the rinsing ultrapure water.
According to the present invention, preferably in the step (a), not only the surface potential of the fine particles is changed but also physical force is applied to the fine particles.
In this preferred embodiment, with the surface potential of the fine particles changed so as to eliminate the electrostatic attractive force acting between the fine particles and the component elements of the ultrapure water supply system or to produce electrical repulsive force between the two, physical force is applied to the fine particles, whereby the fine particles can be removed more easily.
Preferably, in the step (a), the basic solution or the solution of surfactant is caused to flow through the at least part of the ultrapure water supply system at a flow velocity of 0.5 m/sec to 2.0 m/sec.
According to this preferred embodiment, when the basic solution or the solution of surfactant flows through the system, the solution contacts with the fine particles in the system and, in addition, applies physical force thereto, thus promoting the removal of the fine particles.
Alternatively, in the step (a), with the basic solution or the solution of surfactant kept in contact with the at least part of the ultrapure water supply system, the solution is applied with small-amplitude vibration. According to this preferred embodiment, the solution contacts with the fine particles in the system and changes the surface potential thereof, and in addition, small-amplitude vibration of the solution is transmitted to the fine particles to apply physical force thereto, whereby the removal of the fine particles can be promoted.
Preferably, the basic solution is an aqueous solution of ammonia or ammonium salt, or an aqueous solution of alkali metal hydroxide, or a mixture of an aqueous solution of ammonia or ammonium salt and an aqueous solution of alkali metal hydroxide. Alternatively, the basic solution may be pure water or ultrapure water in which alkaline gas is dissolved.
The basic solution used in this preferred embodiment can change the surface potential of the fine particles without fail, thus facilitating the removal of the fine particles.